1. Field of Invention
This invention pertains to amplification of an optical signal with a waveguide made from glass having phonon energy of less than about 350 cm.sup.-1 and doped with dysprosium.
2. Description of Prior Art
There is a need for 1.3 .mu.m fiber optic amplifiers for telecommunications to enable high speed optical networking in local area networks for both military and commercial applications. Currently, a large proportion of the embedded plant is at 1.3 .mu.m. Transoceanic submarine links utilize erbium doped fiber amplifiers based on silica fibers operating at 1.55 .mu.m which corresponds to the wavelength of minimum loss in silica. However, the local area network applications require amplifiers at 1.3 .mu.m for high bandwidth applications since this corresponds to the wavelength of zero dispersion in silica.
Currently, the telecommunication industry uses bulky electronic repeaters at 1.3 .mu.m which convert the optical signal into an electronic signal which is amplified and then converted back into an optical signal via diode lasers. These repeaters are cumbersome, are detectable, and require high-cost maintenance. The telecom industry is actively involved in the development of compact fiber optic based amplifiers.
Silica fiber is not an appropriate host for 1.3 .mu.m emission since the rare earth ions of praseodymium and dysprosium undergo multiphonon relaxation from their respective excited state, i. e., from .sup.1 G.sub.4 to .sup.3 F.sub.3 in praseodymium and from .sup.6 H.sub.9/2, .sup.6 F.sub.11/2 to .sup.6 H.sub.11/2 in dysprosium. This effectively quenches the 1.3 .mu.m emission rendering the device impractical. The phonon energy of silica is about 1100 cm.sup.-1.
Much work has been performed in doping rare earth ions in lower phonon energy glasses such as fluoride glasses with phonon energies of greater than 500 cm.sup.-1, and more recently, in sulfide glasses with phonon energies of greater than 400 cm.sup.-1. The work in fluoride fibers has actually led to a commercial product, i. e., a praseodymium doped fiber amplifier operating at 1.3 .mu.m. However, this product has not found widespread acceptance since the quantum efficiency is very low at less than 5%, signal gain is small at about 0.08 dB/mW, and the pump power requirements at 1.02 .mu.m are extremely high at about 440 mW. The latter property leads to short diode lifetimes which inevitably adds to the cost and lack of reliability of the system. Furthermore, the chemical durability of fluoride glasses is poor compared to silica based fibers which have to meet long term reliability of greater than 15 years. All these characteristics contribute to the unfavorable response by the telecom industry to the use of praseodymium doped fluoride fiber amplifiers. While praseodymium does not work well in fluoride glasses, the 1.3 .mu.m emission of dysprosium is even weaker in these glasses since it is multiphonon quenched.
On the other hand, sulfide glasses possess lower phonon energies than fluoride glasses and so consequently, have been doped with praseodymium. The glass compositions cited in the literature are based on Ga-La-S and Ge-Ga-S. The expected gain is proportional to the value of .sigma..sub.e .tau. which represents the product of the measured emission cross-section (.sigma..sub.e) and the lifetime of the excited state (.tau.). The values of .sigma..sub.e .tau. obtained for Ga-La-S, Ge-Ga-S and the fluoride glasses doped with praseodymium are 250, 479 and 39 (.times.10.sup.-26 cm.sup.2 s), respectively. This indicates that the expected gain is about an order of magnitude higher than that obtained for praseodymium doped fluorides for the same pump power. In other words, the praseodymium doped sulfide glasses require about 10 times less pump power for the same gain. Quenching from the .sup.1 G.sub.4 level in praseodymium still occurs to the .sup.3 F.sub.3 level but to a lesser extent so that the quantum efficiency for the 1.3 .mu.m emission is higher at about 30%. However, using the rate equation model for a 4-level system, it would appear that very low fiber losses are necessary in order to realize a practical amplifier device. For example, a loss of about 1 dB/m is required at around 1.3 .mu.m to attain a gain of greater than 20 dB using a fiber length of over 10 meters containing 500 ppm of praseodymium. To date, the lowest loss reported in a multimode sulfide fiber over several meter lengths is only about 0.5 dB/m and occurs at longer wavelengths, typically at wavelengths greater than 2 .mu.m. The loss at 1.3 .mu.m is closer to 1 dB/m, at best. The problem is further highlighted by the fact that a single mode fiber of about 2 .mu.m core diameter is needed for a fiber amplifier. The lowest loss reported to-date for a single mode sulfide fiber is greater than 1 dB/m, measured on a 1 meter length at long wavelengths exceeding 2 .mu.m. Practical realization of a fiber amplifier at 1.3 .mu.m requires 10-20 meter lengths of low loss praseodymium doped sulfide fiber which is difficult and cost intensive task to achieve.
Dysprosium has also been doped in sulfide glasses. For instance, excited state lifetime of only 38 .mu.s for dysprosium in a Ge-Ga-S glass pumped at 810 has been measured. The emission was very weak. As.sub.2 S.sub.3 glass doped with dysprosium did not show any fluorescence at 1.3 .mu.m. More recently, Ge.sub.30 As.sub.10 S.sub.60 and Ge.sub.25 Ga.sub.5 S.sub.70 glasses doped with 0.2-0.5 weight percent dysprosium showed very weak fluorescence at 1.3 .mu.m when pumping at about 808 nm. In another example, fluorescence was not observed in As.sub.2 S.sub.3 glass containing 0.1 weight percent dysprosium although in As.sub.2 S.sub.3 glass containing 1.7 mole percent iodine, in Ge.sub.30 As.sub.10 S.sub.60 glass, in Ge.sub.25 Ga.sub.5 S.sub.70 glass and in Ge.sub.35 S.sub.56.5 I.sub.8.5 a weak fluorescence was observed at 1.3 .mu.m with a measured lifetime of less than 38 .mu.s, leading to a quantum efficiency of less than 20%. Ga-La-S glass has been doped with 500 ppm of dysprosium and fluorescence was observed at 1.3 .mu.m with a measured lifetime of 59 .mu.s, leading to a quantum efficiency of about 29%. This was achieved by pumping at 815 nm as well as 914 nm.
It appears that to date, the best rare earth ion dopant is praseodymium in a sulfide host for a potential fiber amplifier at 1.3 .mu.m for telecommunications. However, the weak absorption cross-section for the .sup.1 G.sub.4 level necessitates the use of longer fiber lengths and, therefore, low fiber losses are absolutely critical to enable this application.
Those skilled in the art have ruled out the use of lower phonon energy hosts or glasses such as selenide glasses, whose phonon energies are less than about 350 cm.sup.-1, doped with praseodymium and dysprosium for potential 1.3 .mu.m fiber optic amplifiers. This is because these ions are typically pumped at 1.02 .mu.m and 0.815 .mu.m (and 0.915 .mu.m ), respectively. It is well known that the electronic edge, i.e., the short wavelength edge, is shifted to longer wavelengths in selenide glass hosts and so absorption by the electronic edge would be significant at the pump wavelengths used thus far, namely 1.02 .mu.m in praseodymium and 0.815 .mu.m or 0.914 .mu.m in dysprosium. In addition, these small band gap materials exhibit a weak absorption tail which extends into the infrared. Consequently, it is widely believed that the emission at 1.3 .mu.m would also be appreciably absorbed by the selenide host matrix, thereby making such amplifiers impractical.
In addition to the negative sentiments regarding the optical performance at 1.3 .mu.m, it is widely believed that low phonon energy hosts, such as the selenide glasses (&lt;350 cm.sup.-1) possess poor solubility of rare earth ions which leads to clustering and crystallization, especially during fiber drawing. This has a detrimental effect on the fluorescence at 1.3 .mu.m. Such clustering has been attributed to the lack of an appropriate site for the rare earth ions which arises from the difference in bonding character between the glass forming selenides and the rare earth precursors. In other words, the solubility of rare earths in low phonon energy selenide glasses is low since the glass structure consists of predominantly covalent bonding character while the rare earths prefer to form predominantly ionic bonds. Also, the refractory nature of rare earth selenides makes it harder for them to dissolve in the glass melt.
Optical waveguides come in different shapes and sizes, and examples include fibers and planar waveguides. Fibers are usually of circular cross-section and possess diameters of tens to hundreds of microns and lengths typically of a few meters to many kilometers. Planar waveguides typically possess thicknesses from a few microns to hundreds of microns and lengths from a few millimeters to several centimeters. The advantages of planar waveguides are that they are small and compact whereas fibers enable remote/long distance applications. Fiber waveguides are drawn from core/clad preforms or using crucible melting techniques whereas planar waveguides are made by depositing glass on a substrate and then using ion exchange or photochemistry to generate the core/clad waveguide structure.